A semiconductor device is generally manufactured by performing several manufacturing processes on a silicon wafer. For example, a deposition process may be carried out to form a layer of material on the wafer, and/or an oxidation process may be performed to form an oxide layer on the surface of the wafer or to change a deposited layer formed on the wafer into an oxide layer. In addition, a photolithography process may be carried out to form a pattern having a predetermined shape on the wafer, and a planarization process may be performed to planarize a layer formed on the wafer.
Oxide layers are generally formed on a wafer by a thermal oxidation process. The thermal oxidation process is performed using a heat treatment apparatus, such as a vertical diffusion furnace, and by employing an oxygen (O2) gas. For example, a heat treatment apparatus is disclosed in U.S. Pat. No. 6,235,121 to Honma et al., the disclosure of which is hereby incorporated by reference in its entirety.
A conventional heat treatment apparatus may include a vertical processing chamber, a transferring chamber, a wafer boat, a rotating driving unit, and a vertical driving unit. The processing chamber may include a reaction tube, a heating furnace enclosing the reaction tube, and a cylindrical manifold coupled to a lower portion of the reaction tube.
A radical oxidation method using oxygen radicals or oxygen atoms has also been used to form an oxide layer having improved electrical properties on a wafer. For example, the radical oxidation method is disclosed in U.S. Pat. No. 6,358,867 issued to Tews et al.
A radical oxidation method using the conventional heat treatment apparatus described above is generally performed as follows. A plurality of wafers is loaded into the wafer boat of the heat treatment apparatus. The wafer boat is transferred from the transferring chamber into the reaction tube of the heat treatment apparatus by the vertical driving unit. A reaction gas, such as a gas including an oxygen (O2) gas and a nitrogen (N2) gas, is introduced into the reaction tube. Surfaces of the wafers are oxidized by the reaction gas so that oxide layers are formed on the wafers. The wafers are rotated in the reaction tube by the rotating driving unit to increase the uniformity of the thickness of the oxide layers.
Although the oxide layers formed by a radical oxidation method may have electrical properties sufficient for being employed as gate insulation layers of transistors, the thickness of the edge portions of the oxide layers may be thicker than those of the central portions of the oxide layers. This thickness difference may cause several disadvantages in successive processes for manufacturing the semiconductor device.